Before embarking on an elimination diet, some understanding of how foods can cross-react with one another is necessary. Foods derived from two related plants (or two related animals) will have similar proteins. They do not have to look like one another to be alike chemically – our own proteins are 99 per cent the same as those of the chimpanzee and the gorilla, our nearest living relatives. In the same way, the potato and the tomato may look quite different, but the plants they come from are closely related.

If you are allergic to one sort of food, you may show a reaction to food from a related source, because the IgE antibodies that bind to the first protein will also bind to a similar related protein. Cross-reactions also seem to occur in food intolerance, although the mechanism is not understood in most of these cases.

Biologists use various methods to work out how closely two animals or plants are related. There are often further subdivisions within each level, such as the subfamily and the tribe, which are subdivisions of the family.

How is this sort of classification scheme relevant to food sensitivity? Practical experience of thousands of patients suggests that they can cross-react to related foods, although they do not always do so. It also seems, from this collected experience, that the family level in biological classifications is a useful one in deciding which foods will cross-react – although sometimes one has to look at higher or lower levels to understand the cross-reactions that are seen. For example, all cereals are grasses, and belong to the grass family, Gramineae. Some food-sensitive people react to all cereals – to all members of the family. But others react only to wheat or maize, the two most commonly eaten cereals in the West. Many who react to wheat also react to rye and barley, and sometimes oats. If one looks at the classification of the Gramineae, one finds that wheat, rye, barley and oats all belong to the same subfamily, the Pooidae, and wheat, rye and barley are in the same tribe, the Triticeae. Maize is in a different subfamily, and rice in a different subfamily again, so there is less likely to be a cross-reaction between wheat and these cereals. This nicely explains the observation that wheat-sensitive folk are more likely to tolerate rice than any other commonly eaten cereal.


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